Directed energy deposition (DED) is one of the metal additive manufacturing processes that deposits metal by supplying material and a high-power heat source to the substrate. DED can be used for coatings as well as parts building, and interest in DED coating has been increasing. However, the mechanical properties of the deposited material, such as hardness, are influenced by various process parameters. Understanding thermal history is essential for analyzing and optimizing these parameters, as it significantly affects the resulting microstructure and properties. Thermal simulation is an effective method for analyzing thermal history, especially in rotational coatings where experimental measurements are challenging. While several studies have investigated process parameters using experimental and numerical methods, detailed analysis of the hardness of DED coatings based on thermal simulation remains limited. In this study, thermal simulations were performed for rotational coatings of a Fe-based alloy at different process speeds. A detailed analysis of experimental hardness and metallographic structure was conducted based on the thermal history. The simulations used geometrically simplified models by converting the pipe to a cuboid and incorporated element addition and heat input according to laser movement and workpiece rotation. Thermal histories at each point were obtained and comparison of them and hardness distribution revealed that the reduction in hardness under low-speed conditions is not primarily due to changes in the cooling rate to the martensite start temperature (Ms), but rather to higher peak temperatures and longer retention times above the transformation temperature between austenite and ferrite (A3), reducing martensite formation. Furthermore, a gradual decrease in hardness along the coating direction under low-speed conditions can be attributed to the same thermal history. This study demonstrates how thermal history analysis via simulation can deepen the understanding of hardness variation in DED coatings and supports process optimization.